The present invention relates to atomic force microscopes of the systems in which displacements converted from interatomic forces acting between substances by using very small spring elements are detected by a photodetector element as a displacement of reflection light obtained by irradiating laser beams on the spring elements thereby to produce control signals.
An atomic force microscope (Atomic Force Microscope), since it has been invented (Physical Review Letters vol. 56 p930 1986) by G. Binnig et al who are the inventors of STM, has been promoted of researches and studies under expectation as a surface shape observation means for novel insulative substances. The principle thereof is that interatomic forces acting between a detecting tip having a sufficiently sharp point and a sample is measured as a displacement of the spring element attached with the detecting tip, the sample surface are scanned while maintaining the displacement amount of the spring element at constant, so that the shape of the sample surface is measured using, as a shape information, a control signal for maintaining the displacement amount of the spring element constant.
For a displacement detecting means of the spring element, an example referred to as an beam deflection system (Journal of Applied Physics 65(1), 1 p164 January 1989) has been reported wherein the positional displacement of the reflection light obtained by irradiating the laser beam on the spring element is detected by the photodetector element to produce a displacement signal.
FIG. 3 shows an operational principle of the atomic force microscope.
FIG. 4(a) is a conceptual view showing a relationship of interatomic forces versus interatomic distances. When two atoms approximate with each other at a distance of several nanometers or several Angstroms, the so-called Van der Waals force proportional to the minus seventh power of the interatomic distances are generated as an attractive force attracting with each other. When more approximating, then the so-called repulsive force arises suddenly.
FIG. 4(b) is a conceptual view showing a profile where a spring element 3 is being displaced. The conventional atomic force microscope obtains shape data of the sample surface in that the scanning is performed in the inside direction of the sample surface while a sample 1 is being adjusted in the z direction so that displacement amount "x" in the drawing is made constant. For a difference from the so-called tracer type roughness meter which uses a pressure of several milligrams for the sensor during the measurement, the atomic force microscope uses a pressure of as small as equal to or less than a microgram and has a very higher resolution and the like properties in spite of a narrower observation range than that of the roughness meter.
FIG. 3 is a schematic view showing the atomic force microscope of this conventional optical lever type. The spring element 3 is attached with a detecting tip 2 for limiting an interaction with the sample 1 into a minute portion to constitute a detector for detecting a very small force. The sample 1 is supported by a fine motion element 4, and a fine motion mechanism 4 is further supported by a coarse motion element 5. The detecting tip 2 is three dimensionally driven so as to be positioned at an interatomic force measurement region of the surface of the sample 1. The spring element 3 is attached on a frame 30, which fixes a coarse motion mechanism 5, where the example 1 is three dimensionally driven relative to the tip end portion of the detecting tip 2 by the fine motion element 4. That is, the sample 1 is scanned on the flat surface of the example 1 at a high resolution by the fine motion element 4 while adjusting a distance between the tip end portion of the detecting tip 2 and the surface of the example 1. Since a fine motion amount equal to or less than a nanometer is required, a good amount of cases use piezoelectric elements as a fine motion element. The fine motion element 4 is fixed on the coarse motion mechanism 5 for performing the rough positioning of the example 1 and the spring element 3.
The rear surface-side of the spring element 3 is provided thereon with a displacement detecting system for detecting a displacement amount of the spring element 3. First, a light emitted from a semiconductor laser 6 is converged by a lens a8 and adjusted by an optical axis adjusting means 22 so as to be irradiated on the rear surface tip end portion of the spring element 3. The semiconductor laser 6 is driven by a laser driver 18. The spring element 3 is provided with coating for raising a reflection index. The reflected light is converged by a lens b9 on a two or four segmented position sensitive detector (PSD) 11. In case where, as a photodetector element, for example, two segmented photodetector 11a is used, an adjustment is made so that the light is incident uniformly on the previously splitted element, and then a difference signal is taken from between the two-segmented elements. When the spring element 3 is tilted by being pressed by the sample 1, an optical spot on a light receiving surface of the photodetector 11a is moved proportional to a tilt of the spring element 3, thus one-side output of the splitted elements increases, and the other-side decreases. As a result, the differential output comes proportional to the tilt of the spring element 3, i.e., the displacement. This displacement signal, which is taken into a servo system 20 after being amplified by a differential amplifier 19, is converted into a control signal fed to the fine motion element 4 and the coarse motion mechanism 5, and controlled for obtaining a constant distance between the example 1 and the spring element 3. The servo system 20 is coupled to computer 21.
However in the conventional optical lever type of atomic force microscope, because a driving system is employed to drive the sample by the fine motion element 4, an observation comes difficult with the decrease of a resonance frequency of the fine motion element if a larger sample intends to be observed. A small size of the fine motion element itself, for example, a diameter of 30 millimeters at most when a cylindrical type piezoelectric element is used, provides difficulty to physically attach the sample, and this requires cut of the sample, for example, for observing a semiconductor wafer and an optical disk substrate. For this reason, a drawback has been arisen because there can not be taken an advantage of non-destructive observation existing in the atomic force microscope.
Since the system for driving the sample by the fine motion element is used, a load mass of the fine motion element is varied at every time of measurement and a problem is that a control characteristic and a measurement speed are not constant.